In the course of producing oil and gas wells, typically after the well is drilled, the well may be completed. In many instances, in order to complete the well the well may be cased. In certain instances the process of installing casing into the wellbore may begin with a cement float shoe threaded to a coupling and the coupling is threaded onto the first joint of casing. Typically, each joint of casing is about thirty feet long with a pin connection on each end. A coupling typically has a box connection on both ends. As the casing is lowered into the well a coupling is attached between each joint of casing to allow the joints of casing to be threaded together.
Once the casing is located at the appropriate position in the wellbore cement may be pumped into and down the interior of the casing. The cement may both anchor the casing into position as well as isolate the hydrocarbon bearing formation from another section of the same formation or from other formations that are penetrated by the same wellbore. Once the cement reaches the cement float shoe the cement flows out of the casing and then into the annular area outside of the casing between the casing and the wellbore. The cement is forced into the annular area generally until the annular area is filled with cement. Once an appropriate amount of cement is pumped into the casing a wiper plug may then be used push the cement out of the casing and to eliminate as much of the remaining cement as possible from the interior of the casing.
Generally the next step in completing the well, after the cement is allowed to set or cure is to form ports in the casing to allow the fluids from the formation into the interior of the casing. One of the current methods of forming the ports in the casing is known as plug and perforate. Typically, fracturing operations in oil and gas wells today involve fracturing plugs. Fracturing plugs are typically deployed on electric line in combination with perforating guns and a plug setting tool. The operator uses the weight of the fracturing plug, perforating guns, and plug setting tool to lower the assembly to the desired location in the well. For highly deviated and horizontal wells, gravity will not provide the assistance to move the assembly out in the horizontal lateral section, therefore many operators use a ‘pump-down’ technique where fluid is pumped into the well above the assembly thereby forcing the assembly into the well and while allowing the electric line to unspool. The operator monitors the length of electric line deployed to determine where the assembly and in particular the fracturing plug is located in the well. Once the assembly reaches the desired setting location, the surface pumps are stopped and the fracturing plug is set by sending an electric signal down the electric line which fires the setting tool. The setting process also releases the plug from the setting tool.
Once the fracturing plug is set within the wellbore, the operator may then perform a second operation to perforate the casing allowing access the adjacent formation. The perforating guns utilize specially designed explosives to perforate the casing allowing access to the adjacent formation above the fracturing plug's pre-determined location. After firing the perforating gun, the operator may move the perforating gun and setting tool up the casing and the perforating gun is again activated. The process may be repeated until all of the perforating gun's sections have been utilized.
After firing all of the desired sections of the perforating gun the operator typically retrieves the electric line and spent perforating guns. Once the electric line and spent perforating guns are removed from the wellbore the operator may then rig up the surface piping to conduct pressure fracturing operations through the new perforations and into the formation with the fracturing plug that was placed into the casing isolating that portion of the casing below the fracturing plug and allowing only the portion of the formation that was accessed by the perforating gun to be fractured
Formation fracturing occurs when the fracturing fluid, pressurized from surface using high-pressure pumps, creates a pressure greater than the natural reservoir pressure contained in the formation. This process is commonly known as ‘fracturing the well.’ The pressurized fluid will be diverted by the fracturing plug through the perforations in the casing, therefore the fracturing fluids will be forced out into the formation of interest. Fracturing the reservoir causes cracks and fissures to occur in the rock containing the hydrocarbon, and thus releases the oil or gas to flow into the well bore for easier production.
After fracturing the formation a new perforation assembly is run into the casing where the new fracturing plug is set above the section previously perforated and the entire process is repeated until the desired number of perforations has been completed and the associated portions of the formations have been fractured and treated.
Once the process is complete the fracturing plugs must be removed, typically by milling or drilling out each fracturing plug. It is not unusual for there to be ten or more fracturing plugs that must be removed before the well may be produced. Removing each fracturing plug by milling it out takes a substantial amount of rig time incurring substantial cost. Typically this is accomplished by running a motor and mill assembly on the bottom of either coil tubing or on threaded pipe. Each previously installed fracturing plug may be completely milled up, or removed, from the wellbore. The milling operation may be expensive and can sometimes cost hundreds of thousands of dollars to remove all of the plugs so the well can be placed into production.